Vehicles such as, e.g., self-propelled construction machines, have often one or more hydrostatic drives for driving different components of the vehicle. The vehicles such as self-propelled constructions machines, articlualry wheel excavators, are generally exposed to extreme fluctuations with regard to the vehicle speed to be realized. Known vehicles having a hydrostatic travel drive often include a closed circuit travel system. Such closed circuit travel systems require a large travel pump to generate sufficient flow of a hydraulic fluid during high-speed travel. Vehicles as, for example, wheel excavators or other wheel construction machines as e.g. wheel dozers, wheel loaders, wheel tractor-scrapers, underground mining machines, skid steer loaders, skidders, road reclaimers, industrial loaders, wheel compactors, and feller bunchers, may be operated quite often in a low- or medium-speed travel mode, but quite rarely in a high-speed travel mode. Hence, such hydraulic drives for vehicles, which, for a major operating time, are travelled in a low or medium travel speed mode, comprise an oversized hydraulic pump for the travel system, which may result in high manufacturing costs, and which may have a negative impact on the requisite space within the vehicle, and which may negatively impact the performance of the vehicle.
Some hydraulic circuit construction designs for working machines are known from WO 2009/084853 A2, U.S. Pat. No. 7,581,392 B2, US 2008/0223027 A1, U.S. Pat. No. 7,281,373 B2, US 2010/0094515 A1, U.S. Pat. No. 3,952,510, US 2009/0248259 A, U.S. Pat. No. 3,916,767.
Another hydrostatically driven vehicle with retarder valve is disclosed in U.S. Pat. No. 6,202,783 B1. The disclosed vehicle has a hydrostatic drive and a hydraulic control apparatus. The hydrostatic drive includes a variable displacement hydraulic pump and a hydraulic motor in a closed circuit. An internal combustion engine is configured to drive the hydraulic pump and a variable displacement pump of an open circuit. A retarder valve has a valve and a pressure-limiting valve connected thereto, for throttling the volumetric output flow of the variable displacement pump.
U.S. Pat. No. 6,336,518 B1 refers to a travel assisting hydraulic circuit for hydraulic drive type working vehicles with enhanced general versatility, in which the travelling assisting circuit during travel at a high speed can be configured to be a simple circuit. For this purpose, in a hydraulic drive type working vehicle including a travelling HST circuit having a travelling pump driven by the power of an engine, and a travelling motor driven by the pressurized oil from the travelling pump. The output torque of the travelling motor is transmitted to a drive wheel via a speed reducer. The travelling assisting hydraulic circuit includes an assisting pump driven by the power of the engine, and an assisting motor for outputting the torque generated by being driven by the pressurized oil from the assisting pump to the speed reducer to add the torque to the output torque of the travelling motor.
U.S. Pat. No. 5,946,910 shows a hydraulic circuit for a hydraulically driven working vehicle which allows the vehicle to travel at an almost constant speed during high-speed travelling. The shown hydraulic circuit has a hydraulic travel pump and a working machine hydraulic pump which are driven by the power of an engine for discharging pressurized oil to an HST travel circuit and a working machine driving hydraulic circuit, respectively. Pressurized oil from the working machine hydraulic pump joins pressurized oil from the hydraulic travel pump to effect high-speed travelling, while pressurized oil from the hydraulic travel pump joins pressurized oil in the working machine hydraulic pump to generate a large digging force to effect digging. Again, all hydraulic circuits are designed as open hydraulic circuits.
U.S. Pat. No. 7,604,300 B2 discloses a dump truck or large mining truck, comprising various hydraulic circuits. Two or more hydraulic circuits may be connected to one another by a valve device which is controlled by a control device such that the plurality of hydraulic circuits cooperate in an unloading mode and actuate a dump body together and, on the other hand, the hydraulic circuits work separately in the travel mode so that the second hydraulic circuit and/or any further hydraulic circuit is separated from the dump body and its actuator device.
A hydrostatic circuit for harvesting machines is disclosed in U.S. Pat. No. 6,145,287 A. The hydrostatic circuit disclosed therein comprises a multiple pump apparatus including first and second variable displacement closed circuit pumps. The multiple pump apparatus pressurizes fluid to be subsequently supplied to drive load devices. The first and second pumps each have fluid communication lines connected to load devices through two flow directional valves. Each of the pumps has a swash plate for varying its displacement. The swash plates are of the over-center type, enabling each pump to supply pressurized fluid in either direction, thus providing both forward and reverse function to the load devices. The first pump has its fluid communication lines connected to fluid communication lines between first and second 4/2 directional valves (four-port two-position directional valves). The opposite side of the first directional valve has fluid communication lines connected to a load device which drives a harvesting mechanism, in this case a rotor motor. The opposite side of the second flow directional valve has fluid communication lines connected to the circuit for driving the load device for the propel function, in this case the propel motor(s). In this way, the combined flow from the two pumps enables the vehicle to attain high-speed operation for roading without the need for a single large fluid displacement propel pump to support this performance requirement. Nevertheless, the number of flow directional valves may result in a higher susceptibility to damage. Moreover, the necessary number of flow directional valves increase manufacturing costs.
The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.